1. Field of the Invention
This invention relates to the field of aerospace, and in particular to the field of spacecraft system architecture and design.
2. Description of Related Art
All spacecraft have substantially the same basic requirements: power, communications, guidance, navigation, control, and command and data handling. Conventionally, the design of a spacecraft, such as a satellite system, is effected by partitioning the spacecraft into two independent sub-systems: a payload system and a transport system. The payload system comprises the mission-specific equipment, such as a collection system that collects data in a research satellite, a relay system that retransmits signals in a communications satellite, and so on. The transport system, or "bus", comprises the equipment required to effect the mission in space, including: the power generation and storage system, the attitude determination and control system, the command and data handling system, the communications system, and the infra-structure and super-structure to support each of the components of each system.
Although the functional partitioning of tasks between payload and transport systems provides the desired degree of functional independence for effective system design, the physical constraints inherent in spacecraft design often forces a structural dependence that minimizes the advantages that can be gained by this functional partitioning. For example, spacecraft missions often involve the collection of data. The arrangement of the solar panels that provide power to the spacecraft, the design of the attitude control system, and other spacecraft specific designs will be dependent upon the particulars of the collection equipment. If the mission is to visually collect data related to the earth's surface, for example, the solar panels must be arranged so as not to obscure the view of the earth, and the spacecraft must be controlled to orient the visual collection device toward the earth. Conversely, if the mission is to measure the effects of weightlessness on crystal growth, the solar panels can be placed anywhere on the exterior of the spacecraft, whereas the spacecraft propulsion and control system must be designed to minimize acceleration in any direction.
In like manner, the demands on spacecraft sub-systems, such as the communications system and the power generation systems, are substantially affected by mission-specific requirements. Typically, the payload and transport systems are designed using a specified allocation of power and bandwidth among the components. As the designs of the payload system and the transport system progress independently, issues arise when the actual requirements exceed the anticipated requirements. When such issues arise, a choice typically must be made between increasing the allocation of resources to the component requiring the additional resources, or decreasing the capabilities of the component to conform to the specified allocation. Increasing the allocation often requires a redesign of the transport system components that provide the resource, while decreasing the capabilities to conform to the specified allocation often requires a redesign of the payload system. Often, the determination of the actual requirements of each component or sub-system does not occur until a substantial portion of each system is designed. As is known in the art, the cost of design changes, in time, effort, and materials, typically increases exponentially with respect to the degree of completion of the design, and there is a very high cost associated with changes that occur late in the design cycle.
The overall structure of the transport system is also substantially dependent upon the payload requirements. The transport system typically provides the mechanical load-bearing structure to contain each of the components and sub-systems. As in the case of power and bandwidth allocation, space and weight are allocated among components. When an actual requirement exceeds the allocation, a redesign of the transport or payload system, or both, is typically required.
The above noted interdependencies, and others, between the payload system and the transport system are often a major contributing factor to the high cost, in time, effort, and material, of conventional spacecraft development programs. Because of the interdependencies imposed between the payload and transport systems, costly redesigns are often required late in the development cycle, when actual requirements and dependencies become known. Because of the interdependencies imposed between the payload and transport systems, the re-use of systems or sub-systems among spacecrafts having different missions is a sought-after but often unachievable goal.